View Full Version : A320 alternate braking system.

23rd Apr 2010, 12:54
Appreciate any help on the following:

Is it correct to say that when we use 1). Alternate Braking WITH antiskid, we use the yellow hydraulic system and can make as many brake applications as we require at any pressure using the foot pedals.

When we use 2). Alternate Braking WITHOUT antiskid, we lose yellow hydraulic and we must use accumulator 1000psi, 7 applications max.

If this is the case, why does antiskid determine if we have to use accumulator or not. why can't we still have yellow accumulator pressure for braking even if we have no anti skid (inop) for example?

To put it simple, I understand the Normal Braking system/principle and I understand the Parking Brake application. I just don't fully understand during Alternate Braking, when are we to use 7 apps (accumulator pressure) and when do we not need to, and why.

Appreciate your feedback.

Thanks guys,


23rd Apr 2010, 18:02
Ok, let's have a go at this. Doubtless others with access to FCOM will contribute.

1. Yes - no problem.

2. Depends why you are downgraded to Alt Brks. If you are unfortunate enough to have lost GRN + YLO, the you are down to accumulator braking ie 7 applications max 1000psi. But if you are there for some other reason but still have YLO (double BSCU fail?) then you have as much braking as you like but may have to modulate to 1000psi yourself. The ECAM Status Page and the ECAL Wheel Page will tell you what you have.

Over to others!


23rd Apr 2010, 18:26
Hi C4,

In the old days - we would physically turn the antiskid off when we only had accumulator pressure available. This was to prevent the antiskid system releasing & then reapplying the brakes, thus depleting the accumulator too quickly.

Now, Airbus logic will turn the antiskid off when you are down to accumulator pressure only. Thus you have about 7 brake applications available.

If you have Y hyd system pressure available, then you can make as many applications as you like with antiskid still working. The difference with Y system brakes is that there is only 1 antiskid sensor per tandem pair of wheels - and both wheel brakes on the same tandem pair are released when a skid is sensed. Thus it's not as effective as Green Sys brakes, where each individual wheel is sensed and each wheel brake is modulated individually.

23rd Apr 2010, 18:54
thanks guys for your input, i really appreciate it and it makes things clearer.

rudderrudderrat, you said...
'If you have Y hyd system pressure available, then you can make as many applications as you like with antiskid still working.'...... -is this at 1000psi also?

- is 1000psi the standard pressure we put on the breaks when we push brake pedals, whether we are on accumulator only or even if we still on Y hyd?

thanks for your help guys! :ok:

23rd Apr 2010, 19:15
Hi C4,

With antiskid working, you can probably apply up to 3,000 psi to the brakes, (as shown on the Y hyd brake pressure gauge with parking brake set). The antiskid system will release the brake pressure, before wheel lock and tyre destruction.

Without antiskid, you are advised to limit the brake pressure to 1,000 psi - in case you lock the wheel and destroy the tyre.

Some later modified aircraft now have an automatic maximum brake pressure limiter of 1,000 psi - (even if you press the pedals harder) when antiskid is off.

23rd Apr 2010, 20:02
got it!

Thanks for your time mate.